1. Field of the Invention
The present invention relates to an active matrix substrate including a plurality of spaced-apart signal input terminals for providing connection to the driver, a liquid crystal display apparatus having the same, and a method for manufacturing the same.
2. Description of the Prior Art
With the recent advent of the information-oriented society, there is a rapidly-increasing demand for notebook computers, PDAs (personal digital assistants), car navigation systems, and the like. Accordingly, displays for use with these new technologies, such as liquid crystal display apparatuses, have been actively researched and developed. Among various types of displays, the most widespread is the liquid crystal display apparatuses. A liquid crystal display device used in a liquid crystal display apparatus selectively drives picture element electrodes arranged in a matrix pattern to form a display pattern on the screen. Specifically, a voltage is applied between each picture element electrode that is being selected and a counter electrode opposing the picture element electrodes to optically modulate a portion of a display medium such as a liquid crystal material that is present between these electrodes, whereby a display pattern is visually recognized. With an active matrix driving method, among other methods for driving the picture element electrodes, independent picture element electrodes arranged on an insulative substrate are driven by turning ON/OFF the switching device provided in each picture element electrode. The substrate on which the picture element electrodes and the switching devices are provided is commonly called an “active matrix substrate”. As the switching devices used for the selective driving of the picture element electrodes, thin film transistors (hereinafter referred to as “TFTs”), MIM (metal-first insulating film-metal), etc., are commonly known in the art.
More recently, flexible displays have been actively researched and developed, aiming at reducing the weight of a display module, improving the durability thereof, etc. For flexible displays, plastic and stainless steel boards have been used as the mother board of the active matrix substrate.
FIG. 17 illustrates a liquid crystal display device 12 using a common conventional active matrix substrate 9. In the liquid crystal display device 12, a counter substrate 8 including color filters, counter electrodes, etc., (not shown) formed thereon, and the active matrix substrate 9 including source bus lines, gate bus lines, pixel electrodes, switching devices, etc., (not shown) formed thereon, are provided so as to oppose each other, with a liquid crystal material (not shown) sealed within the gap between the substrate 8 and 9. In the frame area of the active matrix substrate 9, gate signal input terminals 10a are provided at one end of the gate bus lines, and source signal input terminals 10b are provided at one end of the source bus lines.
FIG. 10A is a plan view illustrating signal input terminals 10 of the conventional active matrix substrate 9, FIG. 10B is a cross-sectional view taken along line XB—XB of FIG. 10A, and FIG. 10C is a cross-sectional view taken along line XC—XC of FIG. 10A.
As illustrated in FIG. 10A to FIG. 10C, in the conventional active matrix substrate 9, a plurality of line terminals 2 made of a gate metal or a source metal are provided on an insulative substrate 1, an insulating film 13 is provided thereon with contact holes 7 therein corresponding to the line terminals 2, and terminal pads 4 are formed so as to cover the corresponding contact holes 7. Each terminal pad 4 is electrically connected with the corresponding line terminal 2 to form the signal input terminal 10.
A method for manufacturing the conventional active matrix substrate 9 will be described with reference to FIG. 11A to FIG. 11C (corresponding to FIG. 10A), FIG. 12A to FIG. 12C (corresponding to FIG. 10B, taken along line XB—XB of FIG. 10A), and FIG. 13A to FIG. 13C (corresponding to FIG. 10C, taken along line XC—XC of FIG. 10A).
As illustrated in FIG. 11A, FIG. 12A and FIG. 13A, the active matrix substrate 9 is manufactured by first depositing a metal film made of Cr, for example, on the insulative substrate 1 made of a glass, or the like, by a sputtering method, or the like, and then forming the line terminals 2 by a photolithography method, or the like. A gas barrier layer or a water barrier layer may be provided (not shown) on the insulative substrate 1, as necessary.
Then, as illustrated in FIG. 11B, FIG. 12B and FIG. 13B, the insulating film 13 made of SiNx, or the like, is deposited on the line terminals 2 by a CVD method, or the like, and the contact holes 7 are formed so as to correspond to the line terminals 2 by a photolithography method.
Then, as illustrated in FIG. 11C, FIG. 12C and FIG. 13C, ITO, Al, or the like, for example, is deposited by a sputtering method, and the terminal pads 4 are formed so as to cover the contact holes 7 by a photolithography method.
In the conventional active matrix substrate 9 as described above, when a glass, a silicon wafer, or the like, whose thermal expansion coefficient is small and which undergoes little dimensional change by absorbing water, etc., is used as the insulative substrate 1, the line terminals 2, the contact holes 7 and the terminal pads 4 can be patterned with a high precision, i.e., with substantially no mis-photoalignment, in a photolithography process by an ordinary exposure method using a photomask. However, when the insulative substrate 1 is a so-called “plastic substrate” whose thermal expansion coefficient is larger and which undergoes more dimensional change by absorbing water, etc., than a glass substrate, e.g., a substrate whose main component is polyimide, polycarbonate, polyethylene terephthalate, an epoxy resin, an acrylic resin, polyethylene sulfonate, or a derivative thereof, the line terminals 2, the contact holes 7 and the terminal pads 4 cannot be patterned with a high precision, i.e., with little mis-photoalignment. Therefore, the contact holes 7 and/or the terminal pads 4 may be substantially misaligned with the line terminals 2, as illustrated in FIG. 16A and FIG. 16B.
Specifically, when a panel is produced by using a glass substrate (linear expansion coefficient α: 3.6×10−6/K) as the insulative substrate 1, the dimensional change, i.e., the amount of misalignment, is 0.5 μm or less (5 ppm or less), whereas with a plastic substrate, e.g., polyethylene sulfonate (α: 77×10−6/K), the total amount of misalignment including the thermal linear expansion and the dimensional change by absorbing water is 20 to 50 μm (200 to 500 ppm, or more in some cases). As a result, it is not possible to ensure a sufficient contact area between the line terminals 2 and the terminal pads 4, thereby increasing the contact resistance therebetween. This leads to an increase in the contact resistance between the line terminals 2 and the terminal pads 4, a connection defect therebetween, a driver mounting defect, and thus a display defect.
Moreover, when a plastic substrate as described above is used as the insulative substrate 1 in a structure having the contact holes 7 as in the prior art, the stress is concentrated near each corner of a portion of the insulating film 13 corresponding to the contact hole 7 due to, for example, flexure of the insulative substrate 1 when mounting the driver for driving the liquid crystal panel, thereby causing a crack.